1. Field of the Invention
The present invention relates to a dielectric waveguide and a method of production thereof to transmit a high-frequency electric signal such as a microwave, quasi-millimeter wave, millimeter wave, sub-millimeter wave, and so on.
2. Description of the Related Art
In a frequency band around a millimeter wave, a circuit using a waveguide, namely, a microwave transmission circuit is often used. Generally, the waveguide can be made small in sectional size with increase of the frequency, based on ½ wavelength as a standard. Further, it is known that it is possible to make the size inside the waveguide as small as ∈r−1/2 times by filling a space of the cavity inside the waveguide with a dielectric substance, thus making it a small size. This is called a dielectric waveguide circuit (The basis of microwave circuit and the application thereof p 239–243, by Yoshihiro Konishi, published by Sogo Denshi Shuppan, in 1990). Here, ∈r indicates a relative dielectric constant of the dielectric substance.
In application of these waveguides to a transmission line, a resonator, and so on, signal energy loss in the electromagnetic field causes a problem. Energy loss in an electric conductor and a dielectric material is predominant in the loss described above. Loss in a conductor increases as surface resistance increases, and loss in a dielectric material increases as dielectric loss (tan δ) increases.
A low-loss waveguide using a metal superconductor or an oxide superconductor as a conductor has been researched and developed, and a waveguide type cavity resonator using niobium has become commercially practical in a particle accelerator.
On the other hand, it is known that on the surface of a MgO single crystal (001) (since it is a cubic crystal system, the faces (001), (010), and (100) have substantially the same physical properties), a copper oxide superconducting film being in a strong c-axis crystal orientation is obtained by a plurality of methods such as a sputtering process, a pulse laser deposition (PLD) process and so on. As a method of depositing film, a method can be cited in which the film is deposited under high temperatures of about 600 to 800° C. on a substrate in a reduced oxygen atmosphere. It is known that it is easy to pass a superconductive current along the film surface direction of a c-axis oriented film under a low temperature of the critical temperature Tc or less, compared with an a-axis oriented film. The critical temperature Tc of the copper oxide super conductor is known to be several tens of Kelvins or more, depending on the material.
A waveguide circuit is generally easily made low-loss but easily becomes large in size compared with a planar type circuit such as a microstrip line type, a coplanar type, and so on.
Formation of a super conductive planar type circuit using a substrate on which a copper oxide superconducting film is formed has been researched and developed in many institutions. It is recognized that these circuits can form a low-loss (high unloaded Q) circuit compared with a similar type circuit which uses copper, gold, silver, aluminum or the like which is an ordinary electrically good conductive material as a conductor for a circuit transmission line in a submicro wave and a microwave.